If you are really serious on your giveaway then you should require them to apply here or fill up a registration form so that you can track them and check whether they are still wearing signature or not. Because this campaign will be easily abused in case you REALLY WON the jackpot. Anyone can wear the signature right after you announce that you won the jackpot.

And re-echoing statement above. Most of the user will be recently woke and participate here will be hack or bought account so beware.